Document Protobuf spec validation

README.md

@@ -283,6 +283,29 @@ after parsing is faster. For workloads that read every field the costs
 invert; for workloads that read a handful of fields from large messages roto
 wins.
 
-## Literature
+## Protobuf Spec Validation
 
-https://protobuf.dev/programming-guides/encoding/
+The goal is to validate roto's implementation against the Proto3 specification.
+
+### Supported Features
+
+- **Scalar Types**: `double`, `float`, `int32`, `int64`, `uint32`, `uint64`, `sint32`, `sint64`, `fixed32`, `fixed64`, `sfixed32`, `sfixed64`, `bool`, `string`, `bytes`.
+- **Messages**: Top-level and nested message definitions.
+- **Enums**: Enum definitions with `from_i32` conversion.
+- **Field Labels**: Singular and `repeated` fields.
+
+### Unsupported Features
+
+- **`oneof` Fields**: Not currently supported in code generation.
+- **`map` Fields**: Not currently supported.
+- **Packed Repeated Fields**: `roto` expects individual tags for all repeated elements; it does not support the packed encoding used for scalar numeric types in Proto3.
+- **Reserved Fields**: `reserved` statements are ignored.
+- **Services**: `service` and `rpc` definitions are ignored.
+- **Options**: Field and message options are ignored.
+
+### Tasks
+
+- [x] Analyze `roto/codegen` to determine which protobuf constructs are supported during code generation.
+- [x] Analyze `roto/runtime` to determine which wire types and protobuf types are supported during reading and writing.
+- [x] Compare findings with the Proto3 spec (https://protobuf.dev/reference/protobuf/proto3-spec/).
+- [x] Document supported and unsupported features in the README.